Probes of the aforementioned type have been successfully operating for decades for the measurement of small and extremely small magnetic fields or field gradients in a great variety of technological and research applications. One important application for these probes is as a magnetic detector, which, due to the disturbances of the geomagnetic field, are capable of assisting in the determination of the presence and location of hidden ferro-magnetic bodies causing such disturbances, as, for example, bomb shells, sunken ships and the like. Recently, the aforementioned probes are being increasingly used in the exploration of space.
Various embodiments of these probes are known. In "Zeitschrift fur Metallkunde" ("Magazine for Metal Arts"), volume 46 (1955), issue 5, Dr. Forster describes a probe having two probe cores and, respectively, one excitation winding and one receiver winding for each probe core. Also, U.S. Pat. No. 2,752,564 discloses a probe having two probe cores and two windings encompassing one of the probe cores, respectively, which windings, together with a symmetrical excitation current source are arranged in a bridge circuit. In FIG. 4 of this same patent is also shown a probe which has only one probe core with one winding.
Irrespective of the aforementioned embodiments, the material, shape and pretreatment of the probe core are of the utmost importance for the proper functioning of a probe as well as for the uniformity of its properties. This applies in particular to high resolution, sensitive probes. Hence, good probe cores are made from highly permeable, almost non-hysteretic alloys with low remanence.
Although the basic effect for harmonic probes was discovered in nickel wires, oblong, thin nickel strips are almost exclusively used as probe cores for qualified probes. This is mainly due to the fact that in wire-shaped cores eddy currents are caused by the alternating excitation field to an undesirably high degree. Due to the relatively high temperature dependence of electrical conductivity, these eddy currents cause an unacceptable temperature variation of the probes. In order to keep the resulting eddy currents small, it was therefore obvious to use relatively thin probe cores. On the other hand, the resolution of the sensitive probes is limited by the noise resulting from so-called Barkhausen discontinuities which occur in the probe core due to irregularities during the tipping of the magnetic moments of the Weiss' domains and which produce voltage signals at the probe output. As the probe noise is closely related to the cross-section of the probe core, attempts were made to attain a certain minimum cross-section by means of appropriate lateral dimensions and thus the striplike shape of the probe cores was obtained.
High quality probe cores, in any case, require pretreatment which consists in careful annealing of the core material. On the one hand, this treatment eliminates all mechanical stress conditions in the probe core, and, on the other hand, the core is safely converted to the magnetically soft condition. After annealing, any further contact of the probe core by hand or tools should be avoided as far as possible, to prevent the generation of new stress conditions.
With the increased demands made on harmonic probes, in particular due to the more frequent application in space research, serious disadvantages of the currently used technology became apparent. According to the previous practice, a probe strip is arranged between two textile threads and in most cases is drawn into a hole in a tubule which carries the probe winding, the two textile threads being used for the drawing in. Probe alignment is effected by displacing the probe strip to determine the optimum position and by subsequently fixing the probe strip. Naturally, the easy movability of the probe strip, which is required for the alignment and which is achieved by ensuring an appropriate degree of play, is accompanied by a loss of stability of the strip position. Anyhow, the shape of the strip, in combination with the given suspension of the strip between two threads, admits the possibility of strip distortion, in particular at the edges and corners. Bending of the thin strip cannot be precluded, either. These disadvantages are especially important if high accelerations must be supported during space missions.
In the case of gradient probes, which are used to determine small field variations within strong ambient magnetic fields and which applies to the above mentioned detectors, an additional requirement is that the parallelism of the two differentiating probe halves, i.e., the parallelism of the associated probe strips, must be ensured with extreme accuracy. With previous known probe strips this was not always possible to the desired extent.
A further disadvantage of known probes exists in that, after annealing, the probe must additionally be manipulated during the alignment, since the generation of new strains cannot be avoided in all cases. It is also disadvantageous that strip probes show considerable differences of the demagnetizing factor in the transverse direction to the probe core longitudinal axis. The demagnetizing factor is considerably smaller in the direction of the strip width than in the direction of the strip thickness, so that in the preferred direction of the strip width magnetic field, interferences transversely to the probe axis may result.
In addition, paired probes are characterized by the particularly undesirable fact that two probe strips with exactly the same properties must be found. In practice, this problem is solved by arranging a first probe strip in the opening of the first probe half, and, subsequently, by successively inserting a plurality of probe strips into the opening of the second probe half until a sufficiently small residual voltage is obtained at the probe output. Here it becomes evident that, given the core shape of the probe strip, it is particularly difficult to bring the geometric and magnetic parameters for two probe strips into agreement.